Optimum pH of nutrient
solutions

The key reason for controlling the pH of
nutrient solutions is to make sure that the individual elements in the
nutrient remain soluble.

If certain elements become insoluble,
a white precipitate will form on the bottom of the reservoir. Depending on
the pH, this precipitate may take several hours to form, or even days (Fig
10.1).

This means that whenever nutrient solutions
are stored in a reservoir of either a recirculating or run-to-waste
system, it is important to maintain the pH between 5.0 and 6.0 - with an
absolute maximum of 6.5. This helps ensure that:

a) The individual elements remain dissolved
in the water and are therefore available to be pumped to the root zone -
unlike Fig 10.1.

b) The individual elements are in a soluble
form which roots can readily absorb (Fig 10.2).

pH range limits: pH values above 6.0
are to be avoided more than low values such as pH 4.5 to 5.0. All
essential nutrients are soluble at these lower pH values.

The precise pH at which calcium and sulfate
begin to precipitate is determined by their combined concentrations.
Except for fertilizers ‘low’ in calcium and sulfate, this problem commonly
occurs at ~pH 6.5 when the net* EC is ~2.5 mS; or ~pH 7.0 for 1.5 mS
solutions. As a general rule, to avoid precipitation, higher nutrient
concentrations must be held at lower pH values.

*Assumes make-up water has nil EC.

pH recommendation of 6.2 - 6.3

Although this is a popular recommendation, it
has no scientific basis. It appears to have gained ‘mythological’ status
from the early days of hydroponics when the only cheap means of measuring
pH was the common ‘bromothymol blue’ pH indicator used for testing fish
tank water. Interestingly, the lowest pH value able to be determined by
that indicator is about 6.2. It would appear that this value has
unfortunately become an entrenched recommendation in some sections of the
hydroponic industry.